Perpetual Motion Quiz

On Earth, perpetual motion machines don't stand a chance. You don't even have to look at one to know it won't work. “Oh,” you can say knowingly, “it'll be killed by friction and other dissipative forces.” And most of the time you will be right, which is pretty annoying.

So to make things more interesting, we've put this quiz on Slippery, a planet in the star system Wenwet. On Slippery there is no friction, drag, or viscosity (or any other one-way process that steals energy from machines). You can't walk on Slippery, because your foot can't push against the floor. But you can build perpetual motion machines.

This doesn't mean, however, that every machine you make will run forever. If you try to make a machine that has to do work as well as keep running, it will eventually come to a stop because more energy will be coming out than going in. How good an engineer are you? Can you tell which of these machines will stop on Slippery and which will run forever?

Hammer Wheel

How It Is Supposed to Work

Because there is an odd number of hammers, there are more hammers on one side
of the wheel than the other. This lack of balance makes the wheel turn round.

Would It Keep Running on Slippery?

Provided it went round once, it would keep going forever. The flopping hammers just make it jerky,
like the guy who thought it up. At least that's what one physicist told us. Another one, though,
isn't so sure. The hammers would bounce back and fourth (everything bounces perfectly on Slippery),
so the interaction between hammers and wheel is very complicated and may not repeat from one rotation to the next. If
you can answer this one, you're ready for your Ph.D. in physics.

Perpetual Fountain

How It Is Supposed to Work

The weight of the liquid in the goblet forces the underlying liquid up the tube.

Would It Keep Running on Slippery?

The inventor didn't understand liquid. The weight of the stuff in the goblet
would force liquid only partway up the tube. It would stop when it was perfectly
level with the liquid in the goblet.

Anti-Gravity Pendulum

How It Is Supposed to Work

The bowling ball is a pendulum. Once started, gravity will tend to keep it moving.
Just to make things interesting, we've stuck an imaginary gravity shield under half of its
swing. What happens to the pendulum if you do that?

Would It Keep Running on Slippery?

Every physicst we showed this to hated it. How can you put imaginary physics in a children's
magazine? Well, there's no such planet as Slippery, either, and on Slippery, this gizmo doesn't
just run forever, it gets faster and faster. (On the anti-grav side the pendulum keeps its speed going up, then gets faster going down.)

Magnet Machine

How It Is Supposed to Work

The magnet draws a steel ball up the ramp until it drops through the hole near the top.
It rolls back to the bottom hole, where the magnet draws it back up again.

Would It Keep Running on Slippery?

Yes, maybe if you could just set everything up exactly right: magnet, ramp, ball…. But even then, no way…
too many things bumping into each other…yes…no…oh, forget this one! We're not sure! (But leaning toward no.)

Self–Blowing Windmill

How It Is Supposed to Work

Wind from the bellows blows against the vanes of the windmill, making it rotate.
Its rotation opens and closes the bellows, blowing air against the windmill.

Would It Keep Running on Slippery?

No Way! This machine has a major energy leak. Very little of the wind's energy would go into moving the vanes
of the windmill; the rest would blow away. On Earth, it would be even worse.

Vocabulary

Activity

What role does friction play in the motion of these machines when they are on planet Slippery? What role does friction play when these machines are on Earth?[anno: Friction does not change the velocity of the machine when it is on Slippery, but when the machine is on Earth, friction will slow down the motion of the machine.]

Why do you think the motion of the Anti-Gravity Pendulum would get faster over time if the machine were on planet Slippery?[anno: Answers may vary but could include that the motion of the Anti-Gravity Pendulum would get faster over time if the machine were on planet Slippery because each time the ball swung towards the ground it would gain velocity that would not be reduced by gravity, due to the anti-gravity shield.]

Which of these machines do you think would have the slowest speed on planet Slippery? Why?[anno: Answers will vary. Students might think that the Hammer Wheel might have the slowest speed because its hammers, after one rotation of the wheel, might simply bounce back and forth while the wheel stayed in place. Students might think that the Perpetual Fountain has the slowest speed as once the liquid in the tube reached a certain height, the liquid would stop moving, and the machine would no longer work. Students might think that the Magnet Machine would have the slowest speed because the ball might travel up the ramp at a very slow and bumpy pace. Students might think the Self-Blowing Windmill has the slowest speed since much of the force of the wind is lost in the surrounding air instead affecting the vanes of the windmill.]